Takumi lives in a steel shipping container on the roof of a building, converted into a small apartment. He spends most of his time at home, either playing the fictionalMMORPGEmpire Sweeper Online (in which he is Neidhardt, the most famous and powerful player ever) or watching anime. He has over one hundred "waifus" (figures), his favorite being the character Seira from the fictional animeBlood Tune; he frequently affirms his love of 2-D women and his hatred of 3-D women. He is a borderline hikikomori, having crafted a "minimum attendance shift chart" to ensure that he can attend school as little as possible while still graduating.

When the story begins, he is only known to have even semi-regular interactions with a few people: his bratty younger sister Nanami, his ESO guild partner Grim (whom he speaks to online), the school's resident playboy Misumi, and Seira, whom he has become deluded into thinking she exists and can hear her talking and interact with her as kind of muse. Beyond these, he seems to be quite afraid of interacting with people, exhibiting classic signs of social phobia (such as extreme discomfort with making eye contact).

The game has a unique "delusion trigger" system where the player can, at certain points, opt to view one of Takumi's daydreams/delusions. The player is given a choice between a good delusion (happy, humorous, or H in nature) or a bad delusion (paranoid, violent or tragic in nature). For example, when Nanami visits and berates Takumi for not ever calling home, she takes a swig of soda without asking; the good delusion brings up the concept of an Indirect Kiss, prompting Takumi to compare it to a hentai game scene (at which point it almost does turn into one), and the bad delusion involves Nanami taking a swig of coke, then gagging and coughing up blood and dying. Takumi wonders why that happened, then remembers that he poisoned the coke beforehand. Typically these delusions have no direct influence on the story itself (Takumi simply snaps out of it and the scene continues from where it left off), but seeing certain ones triggers flags that can influence which ending you see. Many of these delusions involve blatant reference to anime (or eroge) tropes.

A fan translation patch for the game exists, but was taken down from the main site due to an agreement with Nitroplus, which unfortunately led to Development Hell as there's no localized version of the game yet, in fact no news after the "agreement" between the original fan translators and Nitroplus were given.

Chaos;Head provides examples of:

Adaptation Distillation: It takes around 20 hours to finish the game (or at least one route of it). The anime adaptation, on the other hand, is little short of six hours long. It hardly takes a genius to figure out that something had to go.

Adaptation Expansion: The anime introduces Ayase's cocytus-delusion, which functions as a freezing attack. Additionally, the events of Episode 8 are anime-only.

Affectionate Nickname: Rimi always calls Takumi "Taku." This is to differentiate between the two people she knows who call themselves "Takumi."

Amplifier Artifact: Di-swords serve as these, allowing the Gigalomaniac to properly make use of their hallucinations. Takumi is the only person who has been evidenced to do them without one.

Angsty Surviving Twin: Yua. Her twin sister Mia died in the "Group Dive," the first First Gen incident. Yua then launches her own investigation into Mia's death. She suspects Takumi is involved in the First Gen murders and pretends to befriend him under the ruse of being a girl otaku. When Takumi finds out her true intentions, Yua ... doesn'ttakeit well.

Bittersweet Ending: Slightly more sweet that bitter, although the real Takumo is dead and Sena's father saved Sena at the cost of his own life, the un-real Takumo finally realizes and confesses his love for Rimi, all the members of his band are still alive, the world was saved and Kozue regained her normal, non-telepathic voice.

The list goes on, believe me - Taboo! (Yahoo!), Slavecard (Mastercard), Grouchosoft Mindorz X Perfect (Microsoft Windows XP Premium... seriously), THREE different McDonald's spoofs (McDeinald's, McDynald's and WacTiKald's), Asos (Asus)... there are easily a half dozen of these in each chapter of the game, and the anime adds in even more.

Cast from Lifespan: Any Gigalomaniac that overuses their powers risks severe damage to themselves. Shogun is very far along with process, thanks to constant use of his powers along with his genetic disorder, and the creation of Takumi put him in a coma for a year.

Character Blog: Part of the website for Chaos;Head Noah features blogs by each of the six main girls and one for Takumi.

The diagrams showing how to control a peron's five senses are also used when the protagonists in Steins;Gate attempt to built a Mental Time Travel machine.

Neidhardt's ESO friend Geji-nee is referenced in Steins;Gate, where she coordinated an attempt by various @channers to locate the IBN 5100, and more concretely in Robotics;Notes, where it's made clear that she's an AI.

Cooldown Hug: Rimi delivers one to Takumi when he starts having a breakdown on a middle of a road.

Counter Attack: When two Gigalomaniacs fight, they can even turn a delusion against the original user.

When Suwa tries to catch Takumi in a delusion to keep him from defending himself, Takumi turns it around on him, forcing Suwa to kill himself.

When Norose uses a delusion that destroys Takumi's sense of himself as a human being, Takumi eventually embraces it, using the delusion to get around No-Self Buffs.

Cruel and Unusual Death: The New Gen murders featuring a guy who was killed by having his blood drained and a guy killed by scooping his brain out with a spoon. However, the 2nd incident easily takes the cake and then some after you get know the circumstances of the murder. Nurse Hazuki, who together with Suwa is the culprit behind all these cases, restrained the unlucky guy on a operating table and proceeded to vivisect him without any sort of anesthesia. And that isn't even the worst part. Remember that an unborn child was inserted into his body? That's Hazuki's baby of whom Suwa is the father. How it got into that guy's body you ask? Well, after slicing the victim open, she - I shit you not - proceeded to cut her own womb open, rip the fetus out and shove into that guys stomach. And she did this without any anesthesia, all while sprouting an Slasher Smile par excellence and mumbling stuff in some sort of trance.

Don't Look at Me!: Takumi has three notable instances of using this: First two times are near the end (or beginning), first one being directed towards The Gaze while the second one is for Rimi. Third one is addressed at pretty much everyone around him after he was set up as an ESP. He never actually says them out loud though.

Downer Ending: Endings A and B. The anime follows AA, the happy ending.

Dysfunction Junction: We'll be here all day if we try listing all of the crazy in this show. It's pretty much a given that the gigalomaniacs are pretty broken at least.

Earn Your Bad Ending: Ending B. It can only be done on a second playthrough, and unlike either Ending A or Ending AA, it can only be accomplished by making a specific decision or Delusion Trigger at 11 points in the game, and you're never told which ones are which.This results in a recut of Ending A from Rimi's perspective, which is even more heartbreaking than the normal bad ending.

11th-Hour Superpower: When Takumi recovers from Gorose's attempt to destroy his whole entity, he has come in terms with being a delusional existence. This means that he can treat himself as one, which in turn makes him indestructible.

Evil Phone: At one point, Takumi gets a phone call from an out-of-service number, where he hears a creepy children's song followed by ambulance sirens and a near-deafening beep. It's the sounds of Noah II starting up.

Finger in the Mail: Happens when Nanami gets kidnapped. Although instead of a finger, it's a whole hand.

First-Person Perspective: In the visual novel. *All* of the visual novel is first-person; Takumi sees everything the camera shows, though it takes him some time before he realizes this and accesses the knowledge.

Formulaic Magic: fun10 × int40 = Ir 2, the equation Takumi discovered in elementary school and the basis of the Noah Project and is referred to as more revolutionary than e = mc2. What the equation actually signifies is never explained in depth.

Go Mad from the Revelation: Sena's mother. She was secretly used as a test subject for creating matter from delusions artificially. Since her second child and Sena's little sister mana died shortly after birth, they created a new baby from artificial delusions and gave it to her. Sena's father was against turning her into a guinea pig, but also didn't want her to realize that her child had died, so he reluctantly approved. The experiment continued for several years, until it was decided that enough results were gathered from it and it will be halted. Sena (who didn't know about the experiment) unfortunately happened to be there when they turned the machines off, causing her mother to freak out upon realizing she's been carrying a corpse and proceeded to tear at her face, eat her child's corpse, and stab her own eyes out with a pair of scissors. All while Sena could do nothing but watch. No wonder she hates her dad.

Government Conspiracy: While it turns out that it wasn't the mysterious 300 man committee behind the events but Norose and a couple of traitors from the committee, they still had a fair few companies and organizations working towards their aims.

How We Got Here: The story starts with Takumi lying in a ruined Tokyo street. Rimi creates a Di-Sword and kisses him just before she stabs him. Subverted in the AA ending, where the sword wasn't Real Booted.

Imagine Spot: Defines Takumi's 'delusions', but it seems as though he doesn't remember them. Some might go into Daydream Surprise territory, except they're obvious to the audience, particularly because you have to trigger them yourself in the game.

Interrupted Suicide: Ayase. Takumi's first conscious use of his delusionary abilities in the game is to create a flowerbed which breaks her fall after she attempts suicide by jumping off the roof of the school. And later on, Takumi himself.

Little Sister Heroine: Several 'Delusion' events play up this angle, such as with the indirect kiss scene.

Loners Will Stay Alone: It's about a guy who is the stereotype for this, complete with an obsession for online gaming, and imaginary wives. He is forced out of it, in circumstances he really really won't enjoy at all.

Memetic Mutation: Applied. After the release of the fourth New Gen murder, which had "Sono me, dare no me?"/"Whose eyes are those eyes?"/"Those eyes... whose are they?" written in blood, the entire city ended up repeating it.

Mind Rape: Most of the delusions that NOAH II causes are psychological attacks, so this is to be expected.

Mind Screw: With Takumi leading the narrative, not only does he not have any idea what's going on, the audience doesn't know much more than he does.

This happens from the gameplay perspective as well. Choosing a delusion trigger will show a flash of images, followed by the screen going slightly blurry, as a sign of Takumi having a delusion. As the game progresses, these indications won't always show up when a delusion is in progress; the player will know only after the game cuts them off in the usual manner. To make things even more confusing, delusion triggers sometimes trigger real events to happen to Takumi.

No-Self Buffs: Gigalomaniacs can't heal injuries with their powers. This is because their powers don't allow them to ignore the pain. Subverted when Takumi learns to turn off his sense of pain.

Otaku: Takumi. Specifically of the Anime sort, though he really likes his MMORPG.

Powered by a Forsaken Child: Creating a Di-Sword for the first time requires a Gigalomaniac go undergo severe psychological trauma. By extension, the perfection of Noah II required creating Gigalomaniacs and taking samples from them.

Psychic Link: Kozue uses this to communicate telepathically. Kind of. It's stated that somehow it is entirely different because it runs off the power of delusions.

Takumi and Shogun have one as well, mainly because they technically are the same person

Rapid Aging: Using the power of delusions apparently causes one to age faster. This is only relevant to Shogun who has a genetic disorder causing him to age faster. As a result he's almost dead of old age at the start of the story at about 17 years old.

Shared Mass Hallucination: One of the central ideas in Chaos;Head, but it runs on the idea in reverse: if you can get more than one person to see the same thing (real or not), it becomes real. Whether it's explained as magic or science depends on the character given the explanation, although the story focuses more on the scientific one.

Tsundere: Sena. She even has a Shout-Out to Shana, Nanami also shows tsundere traits focusing on un-giver strategy

And in one of the delusions, Takumi is called on his phone by an (otherwise nonexistent) incredibly generic Tsundere love interest, whose every second sentence is "It's not that I like you or anything".

I Love Tsundere Music Player (Extras) - "I'll let you listen! But only because it's you."

Twin Switch: The game and anime alike drop a lot of hints that the girl we know as Yua was actually her twin, Mia. The incident that gives her her Di-Sword has her conclude that it doesn't matter which one she is.

Her character route in the Updated Re-release confirms that they indeed switched places right before the incident. Because Mia suddenly didn't feel like attending an otaku meeting she was actually looking forward to (she's quite moody, to be exact), she convinced Yua of switching clothes so she can attend the meeting in Mia's stead.

Unreliable Narrator: The start of the anime is entirely from the perspective of the delusional main character, and he's not sure what's going on.

Everyone is an Unreliable Narrator in this show. Both the pseudo-scientific and pseudo-religious explanations are likely the results of each character's own delusions: Sena, the main source of technobabble, is heavily influenced by the occupation of her father, which ties in directly to her incredibly traumatizing childhood; Ayase, who is the main source of the pseudo-spiritual explanation, is shown reading a book in the hospital entitled "The Knights OF Gradiale," her name for the Di-Sword wielders.

Unwanted Harem: While except for Rimi none of them are after Takumi romantically, elements of the genre are invoked in a fashion. Official art usually depicts Takumi with the six leading girls of the show in a way reminiscent of it as well.

When Takumi is being tortured in the final episode of the series, he slips into a delusion of having a romantic comedy-like relationship with the girls.

Updated Re-release: Chaos;Head Noah, for the XBox 360. Changes to the game include adding routes for each of the girls and changing it to widescreen. They also changed the route system; you now have to finish each route to unlock the True Ending, which can then be directly selected from the Main Menu.

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